This invention relates to an essentially fine grained diamond tool for polishing, cutting, finishing, metal removal, etc. and to a method of manufacturing the tool. More specifically, this invention is directed to employing the Chemical Vapor Deposition (hereinafter CVD) process for preparing the fine grained diamond tool.
The CVD diamond process has been developed for producing diamond tools for use in various applications. For such applications, a free-standing diamond film is typically lapped to a desired thickness and then polished on one side. The diamond film is then generally cut by laser to a variety of shapes such as triangular, rectangular, trapezoidal, rhumbic, etc. depending on the particular application. The diamond shape is then fixed to a standard substrate such as tungsten carbide or whatever substrate may be used for supporting a diamond tool. While brazing is often a common method of attaching a diamond tool to a substrate, other methods are also applicable such as cementing, clamping or any other known or disclosed methods of so attaching a diamond tool to a substrate. When the diamond tool is so attached to a substrate, the diamond tool is then ground on the cutting edges to produce a finished tool which is referred to as a tool insert. Instead of just the diamond edge of the tool insert being ground, the tool insert may preferably be also ground along the cutting edges.
Diamond materials which are used as tool materials are generally classified into monocrystal and polycrystalline diamond structures. Monocrystal diamond, which is excellent in physical characteristics, has the disadvantages of being expensive and difficult in working into a desired shape as a tool insert and has the problem of cleavage.
Polycrystalline diamond structures for tool applications, however, can be generally divided into two types, namely sintered diamond and vapor-deposited diamond. Sintered diamond is obtained by sintering fine diamond powder and a metal such as cobalt (Co) under diamond stable extra high pressure and high temperature conditions. Such sintering techniques are well described in the patent literature and are well known to those skilled in the art. Commercially available sintered diamonds can be produced in fine particle structure such as 10 um or so and have excellent wear resistance with little or no cleavage being observed. However, sintered diamonds contain as much as about 10% of a binder and as such the cutting edge of such a diamond structure may chip during use as a cutting tool. The effectiveness of the diamond tool, upon losing diamond particles, is reduced and the life of the cutting tool is thereby shortened. Also because of the presence of a binder, the heat resistance of the tool is not as good as the monocrystal diamond structure and the diamond layer will thermally degrade at temperatures greater than 700.degree. C. The abrasion resistance of this tool is also reduced due to the lower hardness of the sintering agent which is present in the diamond layer.
The other type of polycrystalline diamond is the vapor deposited diamond structure which has better wear resistance compared to the sintered diamond. It also has better abrasion resistance due to its dense structure which is formed only of diamond without a binder. The vapor deposited diamond is generally prepared by chemical deposition (CVD) by decomposing and exciting a raw material gas which is mainly composed of hydrocarbon such as methane in the presence of oxygen or hydrogen. The gaseous carbon compound is decomposed and deposits on the surface of a substrate to form diamond. The diamond deposited can either be directly deposited on a cutting tool, or can be deposited on a sacrificial substrate which is removed to form a separate diamond layer. This diamond layer can then be shaped and bonded to a suitable cutting tool substrate. The gas is excited by various means such as by a filament heater, microwave energy, radio frequency energy, direct current energy, or other thermal, electrical, or optical methods. Again, the CVD process is well known to those skilled in the art and is well described in the literature.
Typically, CVD diamond has a columnar microstructure. Thus, at the growth surface of a substrate from which the diamond structure grows, a fine grained diamond film is formed with diamond grain sizes of usually less than 20 .mu.m. Once the film has grown to its final thickness of usually about 400 to about 600 .mu.m, the average grain size on the final growth surface is much larger with grain size of about 75 to about 200 .mu.m. The larger grains have a tendency to fracture more easily than the finer grain sizes. Thus, CVD diamond tools are typically oriented such that the substrate side, i.e., the original growth surface of the fine grained diamond structure is facing the work piece. Even so, the problem of the larger grain diamonds is still present on the sides of the tool since significant flank wear will occur during normal usage of the tool.